In previous investigation, we demonstrated that vagal stimulation increased the time constant for collateral ventilation in dog lungs. It has also been reported that local increases in carbon dioxide tension decrease collateral resistance. We plan to investigate in interaction of vagal stimulation and local carbon dioxide tension in the regulation of collateral resistance and time constants are measured in the adjacent lung. Preliminary experiments have demonstrated vagally mediated constriction of collaterals in the lung receiving no histamine. Carbon dioxide tensions will be varied in the lung in which collateral resistance is being measured. In an isolated perfused lung we plan to study the local control of collateral by carbon dioxide. Carbon dioxide tension will be varied within an isolated segment of lung and/or in the adjacent lung parenchyma while we measure collateral resistance in the isolated segment. Preliminary results have suggested a decreasing response following repeated exposure of the segment to carbon dioxide. We will investigate this phenomenon in more detail. Since we have recently demonstrated lobar differences in collateral ventilation in excised dogs lung, we will measure collateral resistance and time constant in different lobes of log and horse lunges.